1. Field of the Invention
The present invention relates to molds for forming composite stacks having cellular cores, and to parts made by means of such molds.
2. Background Information
Document DE-3 444 321 discloses a mold for forming a stack including a cellular core. The mold comprises a punch and a die that are mounted so as to move relative to each other. The stack to be formed is disposed between the punch and the die. Bringing the punch and the die together subjects the stack to hot pressing so as in particular to fix together the various layers of the stack. The punch has a projection having a leading face that is plane and that is provided with pieces of relief having curved faces and projecting from the projection. The die is provided with a cavity suitable for receiving the projection together with the pieces of relief. During pressing, the projection and the pieces of relief crush and deform certain zones of the stack. After forming, the part is substantially plane in overall shape from one margin of the part to the other, the margins having been crushed in the direction of their thickness, the faces of the part having curved zones with small radii of curvature that are different from one face to the other. This type of part can be used in numerous fields of industry, e.g. in the automobile industry. The cellular core, e.g. a honeycombed core, enables the part to be very rigid while being of moderate weight.
Unfortunately, a part formed in that way has a crushed margin whose edge constitutes the periphery of the part is inaccurately and non-uniformly shaped and can be seen on both sides of the part. Such a margin spoils the appearance of the part, it is bulky, it is relatively fragile, and it does not enable the part to be subsequently positioned reliably in abutment against an element in its environment.
An object of the invention is to provide a mold making it possible to form a part of the above-mentioned type while giving it a margin that is acceptable in appearance, that is more compact, that is more robust, and that facilitates subsequent positioning of the part.
To achieve this object, the invention provides a mold for forming a stack including a cellular core, the mold comprising a punch and a die that are mounted to slide relative to each other in a sliding direction;
the punch including a projection having a substantially plane leading face perpendicular to the sliding direction, and at least one piece of relief projecting from said face;
the die being provided with a cavity suitable for receiving the projection, and having a substantially plane end wall parallel to the leading face and suitable for extending facing said leading face, the cavity having a side face and a curved surface zone extending between the end wall and the side face; and
the piece of relief being disposed to extend in the vicinity of the curved surface zone and in the vicinity of the side face when the projection is received in the cavity;
the side face of the cavity being plane and perpendicular to the end wall.
On bringing the two portions of the mold together to form a part, the projecting piece of relief stresses and crushes the composite stack locally so as to force it to match the shape of the curved surface zone of the cavity. Even if the radius of curvature of the curved surface zone is very small, a curved surface zone is obtained on the part that has the same radius of curvature. For example, the curved surface zone of the cavity may be given a radius of curvature that is less than the thickness of the part, so as to obtain a zone on the part that has a radius of curvature less than said thickness.
In addition, the plane side face of the cavity cooperates with the piece of relief to deform locally and cause the margin of the stack including the cellular core to fold down so as to define on said margin a plane external side face that is perpendicular to the general plane of the part and that extends in alignment with the curved surface zone. This configuration of the external side face of the margin hides the edge of the part from view looking from the side of the part, thereby improving the appearance of the part, in particular as seen from said side. In addition, the margin formed in this way is compact. Furthermore, the margin is more robust. Moreover, the external face of the margin has a position that is accurately defined and reproducible, and it can be used to position the part in abutment relative to another element, without or prior to fixing the part to said element.
The side face makes it possible, for example, to bring the part and another element together so that the join is acceptable in appearance because of the curved surface zone, and so as to guarantee correct positioning of the part. Furthermore, the piece of relief defines a trough that is contiguous with the piece of relief.
Advantageously, the piece of relief has a plane external side face perpendicular to the leading face and suitable for extending facing the side face of the cavity when the projection is received in the cavity.
Thus, the mold is suitable for pressing the margin of the stack including the cellular core between the side face of the projection and the side face of the piece of relief, both faces being plane, parallel to each other, and perpendicular to the end wall. The margin can thus be pressed or crushed in a direction that is perpendicular to the general plane of the part, thereby reducing its thickness in this direction. In this way, the internal face of the margin is given a plane accurate shape perpendicular to the end wall, so that said internal face can be used as a reference for positioning an element in the trough defined in the part by said margin, and formed by the piece of relief.
Advantageously, the piece of relief is disposed so that, when the projection is received in the cavity, the piece of relief is separated from the side face of the cavity by a distance less than the height of the piece of relief.
Advantageously, the projection has a plane side face parallel to the side face of the cavity and suitable for coming into contact with said side face of the cavity when the projection is received in the cavity.
Thus, the two faces coming into contact with each other produces a shearing effect, thereby shearing off the surplus portions of stack and in particular the cellular core extending beyond the margin of the part. This shearing-off takes place at the same time as the forming of the part, so that the forming masks at least in part any trace of the shearing on the edge of the part. Since the forming step and the shearing-off step are simultaneous, the method of manufacturing the part is faster.
Advantageously, the die is provided with a piece of relief projecting from the end wall and suitable for extending in the vicinity of the piece of relief on the projection when the projection is received in the cavity, so that the piece of relief on the projection is interposed between the piece of relief in the cavity and the side face of the cavity.
Thus, the piece of relief in the cavity and the piece of relief on the projection co-operate with each other to crush between them the stack including the cellular core. As a result, the cellular core matches more accurately the shape of the first piece of relief on the projection, so that the trough defined by said first piece of relief has a more accurate and reproducible shape. The trough can thus be used for numerous mechanical applications, e.g. for abutting against or receiving an element. In addition, the piece of relief in the cavity defines a second trough in the face of the part that is opposite from the face carrying the first trough.
Advantageously, the projection is provided with a second piece of relief projecting from the leading face and disposed in the vicinity of the first piece of relief on the projection so that, when the projection is received in the cavity, the piece of relief of the cavity is interposed between the two pieces of relief of the projection.
Thus, the second piece of relief on the projection and the piece of relief in the cavity co-operate with each other to press and crush between them the stack including the cellular core. In analogous manner, the cellular core matches more accurately the shape of the piece of relief in the cavity, so that the second trough is formed accurately and reproducibly. The second piece of relief on the projection defines a second trough in the face of the part that carries the first trough.
Advantageously, at least one of the pieces of relief has a plane side face perpendicular to the leading face or to the end wall, and suitable for extending facing a side face and facing one of the pieces of relief when the projection is received in the cavity.
Thus, the trough face defined by the side face of the piece of relief is plane and perpendicular to the plane of the part, and therefore lends itself to numerous mechanical applications.
Advantageously, two of the pieces of relief have respective plane side faces perpendicular to the leading face or to the end wall and suitable for extending facing each other so that, when the projection is received in the cavity, the two faces are separated from each other by a distance perpendicular to the end wall that is less than the height of at least one of the pieces of relief.
Advantageously, at least one of the pieces of relief has a curved surface zone extending between a side face of the piece of relief, and the end wall of the cavity or the leading face, the curved surface zone being suitable for extending facing one of the pieces of relief when the projection is received in the cavity.
Thus, the curved surface zone and the piece of relief suitable for extending facing it are suitable for pressing the stack including the cellular core so as to give a zone of the part a curved shape having a predetermined radius of curvature corresponding to the radius of curvature of the curved surface zone.
Advantageously, said at least one of the pieces of relief has a plane top perpendicular to the sliding direction.
Thus, the top locally crushes the cellular core to a large extent, thereby improving the accuracy of the shape of the associated trough, and in particular giving the trough a flat end wall parallel to the general plane of the part.
Advantageously, said at least one of the pieces of relief has a width less than the height of the piece of relief.
The pressing stress exerted locally on the core is thus increased.
Advantageously, said at least one of the pieces of relief has an edge contiguous both with a top of the piece of relief and with a side face of the piece of relief, the edge having a radius of curvature less than 0.5 times the height of the piece of relief, and being suitable for extending facing a curved surface zone of the mold when the projection is received in the cavity.
The edge thus defines a corresponding curved surface zone between the end wall and the side face of the trough. The space defined by the trough is thus increased without increasing the width or the depth of said trough.
The invention further provides a method of manufacturing a part comprising a stack including a cellular core, by means of a mold of the invention, in which method the part is formed with the mold at ambient temperature.
In above-mentioned Document DE-3 444 321, the mold includes its own heater means, so that the part is formed at a temperature higher than ambient temperature. Unfortunately, because of the shape of the mold, the various zones of the mold are sometimes heated non-uniformly, which can adversely affect the quality of forming of the part. With the method of the invention, that drawback is avoided. The stack may advantageously be pre-heated prior to being inserted into the mold.
The invention further provides a part in particular for a vehicle, the part comprising a stack including a cellular core, the part being plane in overall shape, and having a first face and a second face that are substantially plane and opposite from each other on either side of the part, the part having a curved surface zone contiguous with the first face and with a rim of the part, and a trough extending in the second face in the vicinity of said rim towards the first face, the rim having a plane external side face that is perpendicular to the first face, and that extends in alignment with the curved surface zone.
This part may be obtained by means of the method of the invention.
Advantageously, the trough has a plane internal side face that is parallel to the external side face of the rim, the internal side face and the external side face being opposite from each other on either side of the rim, and the internal side face constituting an internal face of the rim.
Advantageously, the external and internal side faces of the rim are separated from each other by a distance less than the largest thickness of the part as measured between the first and second faces of the part.
Thus, the rim is particularly thin and robust, and it lends itself reliably to numerous mechanical applications.
Advantageously, the external and internal side faces of the rim are separated from each other by a distance less than the depth of the trough.
Advantageously, the part is provided with a trough extending in the first face towards the second face and in the vicinity of the first trough, the trough in the second face being interposed between the rim of the part and the trough in the first face.
Advantageously, the part is provided with a second trough extending in the second face towards the first face in the vicinity of the first trough, the trough in the first face being interposed between the two troughs in the second face.
Advantageously, at least two of the troughs are separated from each other perpendicularly to the faces of the part by a distance less than the largest thickness of the part as measured between the first and second faces of the part.
Advantageously, said at least one trough has a plane internal side face perpendicular to the faces of the part.
Advantageously, said at least one trough has a flat end wall.
Advantageously, said at least one trough has a curved surface zone extending between an end wall of the trough and a side face of the trough, the zone having a radius of curvature less than 0.5 times the depth of the trough.
Advantageously, the rim is an end rim of the part.
The invention further provides a seat for a motor vehicle, said seat including a part of the invention.
The invention further provides a floor or luggage-compartment panel for a motor vehicle, the panel including a part of the invention.
Other characteristics and advantages of the invention appear from the following description of three preferred embodiments and one variant given by way of non limiting.